KALAMAZOO, MI —The probable cause of Enbridge's oil spill in the Kalamazoo River will be announced Tuesday by federal investigators, nearly two years after the largest environmental disaster in the history of Southwest Michigan.

"It is the definitive safety investigation," said Peter Knudson, spokesman for the National Transportation Safety Board, the federal agency leading the inquiry.

The report will explain what the agency believes caused the spill and lay out safety recommendations to help prevent future accidents, Knudson said. About 80 percent of the NTSB's recommendations become adopted.

"The whole point is to get beyond the obvious cause that there was a rupture in the pipe," Knudson said. "How did the pipe get to that place? When did the rupture occur? Why did so much oil spill?"

The NTSB investigation has looked into mechanical factors, staff training, organizational culture, human factors, emergency response and planning. It has focused not just on Enbridge, but every agency involved with the spill, pipeline regulation and cleanup response.

The report will be presented to the full, five-member board Tuesday morning at 9:30 a.m. in Washington D.C. There will be a live video stream of the meeting and MLive/Kalamazoo will have full coverage on the event that morning.

At the end of Tuesday morning's meeting, a list of the NTSB's findings will be available to the public, including the probable cause of the spill and safety recommendations. The full report will be published in a few weeks, once the staff can finalize it and add any changes recommended by the board. Knudson said, however, that everything in the final accident report can be found in documents already available to the public. More than 5,000 pages of documents from the NTSB's investigation were released in May.

The NTSB's investigation into the Kalamazoo spill began days after the rupture was found on July 26, 2010. Enbridge says that more than 800,000 gallons of heavy crude oil spilled from its Pipeline 6B. The EPA, however, said more than a million gallons of oil have been collected. The agency that led the cleanup of the river is conducting its own investigation into how much oil spilled.

The five members of the NTSB are appointed to a five-year term by the president. The NTSB investigates every aviation accident, including the a Colgan Air crash in Buffalo, N.Y., that killed 49 people. It also looks at pipeline accidents, including the explosion in San Bruno, Calif., that killed eight people. Members of the board have had the report on the Kalamazoo River spill for a couple weeks. This will be the first time all the members will come together and discuss it.

There will be a wide range of people watching what comes out of the report, said Carl Weimer, executive director of the Pipeline Safety Trust, a nonprofit organization focusing on pipeline safety.

Members of the pipeline and energy industry will look to see what new regulations may be imposed, Weimer said. Also, state and federal regulators will be watching if the NTSB says the organizations are not verifying enough of what pipeline companies are doing to prevent accidents.

Environmentalists will be waiting to see what the report says about the impact of the oil on the pipe. Many organizations have said the type of oil that was being carried in the pipeline — diluted bitumen or tar sands — caused the pipeline to rupture because it is heavier and, they say, more corrosive. Enbridge has said these claims are not true.

Weimer said he hopes the report clears up what Enbridge knew before spill occurred. Given data and preliminary documents from the NTSB, Weimer said it is generally believed that the rupture was caused by stress corrosion cracking.

The largest preliminary fines issued by PHMSA last week were related to the fact that Enbridge knew about anomalies in the section of its 6B pipeline years before the spill. Weimer said he wants to know if there are any connections between these anomalies and what caused the pipeline to ultimately rupture.

Also, Weimer said a number of engineers in the pipeline industry have told him it is difficult to test for stress corrosion cracks. He said when pipeline companies run tests, they often find hundreds of anomalies in the pipeline that might not add up to anything serious.

"They might not have a connection with one another," Weimer said.

He added that he hopes the report touches on the spill response, as well as some of the long-term health effects of the oil and if more residents should have been evacuated.